[R] lapply to change variable names and variable values

[Simon Kiss]

Thanks both! That solves ! You've made a very happy newbie!
Simon
On 2012-03-12, at 2:52 PM, Sarah Goslee wrote:

> Hi Simon,
> 
> On Mon, Mar 12, 2012 at 2:37 PM, Simon Kiss <sjkiss at gmail.com> wrote:
>> Hi: I'm sure this is a very easy problem. I've consulted Data Manipulation With R and the R Book and can't find an answer.
>> 
>> Sample list of data frames looks as follows:
>> 
>> .xx<-list(df<-data.frame(Var1=rep('Alabama', 400), Var2=rep(c(2004, 2005, 2006, 2007), 400)), df2<-data.frame(Var1=rep('Tennessee', 400), Var2=rep(c(2004,2005,2006,2007), 400)), df3<-data.frame(Var1=rep('Alaska', 400), Var2=rep(c(2004,2005,2006,2007), 400)) )
> 
> I tweaked this a bit so that it doesn't actually create df, df2, df3 as well as
> making a list of them, and so that xx doesn't begin with a . and shows up with
> ls(). I don't need invisible objects in my testing session.
> 
> xx<-list(df=data.frame(Var1=rep('Alabama', 400), Var2=rep(c(2004,
> 2005, 2006, 2007), 400)), df2=data.frame(Var1=rep('Tennessee', 400),
> Var2=rep(c(2004,2005,2006,2007), 400)),
> df3=data.frame(Var1=rep('Alaska', 400),
> Var2=rep(c(2004,2005,2006,2007), 400)) )
> 
> 
>> I would like to accomplish the following two tasks.
>> First, I'd like to go through and change the names of each of the data frames within the list
>> to be 'State' and 'Year'
>> 
>> Second, I'd like to go through and add one year to each of the 'Var2'  variables.
>> 
>> Third, I'd like to then delete those cases in the data frames that have values of Var2 (or Year) values of 2008.
>> 
>> I could do this manually, but my data are actually bigger than this, plus I'd really like to learn. I've been trying to use lapply, but I can't get my head around how it works:
>>  .xx<- lapply(.xx, function(x) colnames(x)<-c('State', 'Year')
>> just changes the actual list of data frames to a list of the character string ('State' and 'Year')  How do I actually change the underlying variable names?
> 
> Your function doesn't return the right thing. To see how it works, it's often a
> good idea to write a stand-alone function and see what it does. For instance,
> 
> rename <- function(x) {
>   colnames(x)<-c('State', 'Year')
>   x
> }
> 
> To me at least, as soon as it's written as a stand-alone it's obvious that
> you have to return x in the last line. You can either use rename() in your
> lapply statement:
> xx<- lapply(xx, rename)
> 
> or you can write the full function into the lapply statement:
>> xx<-list(df=data.frame(Var1=rep('Alabama', 400), Var2=rep(c(2004, 2005, 2006, 2007), 400)), df2=data.frame(Var1=rep('Tennessee', 400), Var2=rep(c(2004,2005,2006,2007), 400)), df3=data.frame(Var1=rep('Alaska', 400), Var2=rep(c(2004,2005,2006,2007), 400)) )
>> xx <- lapply(xx, function(x){ colnames(x)<-c('State', 'Year'); x} )
>> colnames(xx[[1]])
> [1] "State" "Year"
> 
> The same strategy should work for your other needs as well.
> 
> Sarah
> 
> 
> 
> -- 
> Sarah Goslee
> http://www.functionaldiversity.org

*********************************
Simon J. Kiss, PhD
Assistant Professor, Wilfrid Laurier University
73 George Street
Brantford, Ontario, Canada
N3T 2C9
Cell: +1 905 746 7606